1. Field of the Invention
The invention relates to a process of producing semiconductor materials and somewhat more particularly to a process of pyrolytically depositing semiconductor materials, such as silicon, from a reactive gas.
2. Prior Art
German Pat. No. 1,123,300 (which generally corresponds to U.S. Pat. No. 3,120,451) suggests a process for depositing elemental silicon from a gas phase onto a heated surface of a mandrel wherein a reactive gas stream comprised of a mixture of a halogen silane and hydrogen is fed through a hollow reaction housing having the heated mandrel positioned therein and the deposition rate is altered during the course of the deposition process. This prior art process seeks to prevent the deposition of boron present in the reactive gas and to that end suggests that at the beginning of the deposition process, a lesser amount of the reactive gas be fed into the reaction housing and then gradually increasing the reaction gas through-put during the course of the deposition process.
It has been observed that during the course of a pyrolytic or CVD (chemical vapor deposition) process, such as described above wherein a relatively constant reactive gas flow is maintained, the deposition rate relative to a surface unit area on the mandrel and relative to a unit of time progressively diminishes, even when the total amount of, for example, silicon deposited on the mandrel during the unit of time increases proportional to the chronologically actual size of the mandrel surface. Since the temperature of the mandrel surface and the mol ratio within the reactive gas is customarily selected at the beginning of the deposition process so that the rate of deposition is at an optimum, typically at a maximum, the effective deposition rate varies from the optimal rate after the deposition process has run for a period of time because of changing conditions occasioned by the deposition process per se.